Age-related variations in hydroxylation of lysine and proline in collagen

The effect of age on the extent of hydroxylation of lysine and proline both generally and at certain specific sites in collagens from bone, skin and tendon was examined in the chick from the 14-day embryo to the 18-month-old adult. For all collagens there was a marked fall in the overall extent of hydroxylation of lysine with increasing age in both alpha(1) and alpha(2) chains, this fall occurring mostly in a relatively short period immediately after hatching. Hydroxylation of lysine declined to a constant value which, as expected, differed appreciably for each collagen and was considered to be characteristic of the collagen according to its tissue of origin. Hydroxylation of lysine in the N-terminal, non-helical telopeptide region of both alpha(1) and alpha(2) chains, which is important with regard to cross-linking, was relatively high in embryonic collagens. There was, however, a rapid loss of hydroxylation at these sites in skin collagen, occurring both during development of the embryo and in the period immediately after hatching. In contrast some hydroxylation at these sites persisted in bone and tendon collagens and, as judged by examination of peptide alpha(1)-CB1, appeared to reach a constant value in time of about 33% in bone and about 15% in tendon collagen. The actual extent of hydroxylation of lysine in the N-terminal telopeptides and the size of the changes in these values with age appeared to be unrelated to the corresponding whole-chain values, and it is suggested therefore that hydroxylation of telopeptidyl lysine may be under separate enzymic control. The increased hydroxylation of lysine in the embryo was accompanied by only minimal changes in proline hydroxylation, which was very slightly increased in embryonic bone and tendon collagens. Increased hydroxylation of proline in the embryo was, however, readily observed in peptide alpha(1)-CB2 from the helical region of tendon collagen. This hydroxylation was close to the theoretical maximum, in contrast with that observed in post-embryonic tendon, where hydroxylation was incomplete, as in rat tendon (Bornstein, 1967), only four on average, of the six susceptible proline residues being hydroxylated.